This invention relates to an apparatus for and a method of active control for stabilizing multiple vibrational modes of an actuator system.
Servo-controlled actuator systems experience serious problems due to mechanical actuator resonances. These vibrational modes include the natural modes of the actuator and those of any intervening mechanical components. With increasing mechanical complexity, the vibrational modes of any given actuator system become difficult to predict. The problem is further compounded as the operating frequency of the actuator system is increased. The vibrational modes limit the control loop gain of the servo system, reduce bandwidth of the servo system, or both. This causes the controlled element, such as a transducer head, to experience excessive settling time after positioning, poor response to disturbances, poor tracking ability, or any combination of these.
Prior art systems have attempted to ensure stable operation of actuator systems by stabilizing the control loop. This has been done by inserting gain stabilizing filters such as electronic notch filters in the control loop path. These filters are placed in the downstream portion of the control loop to filter out the signal information within the band reject frequency range of the notch and thus help minimize excitation of these actuator vibrational modes.
The technique utilizing notch filters allows the servo control system to effectively ignore lightly damped structural actuator resonances. At the resonances very little control is applied by the servo controller.
The drawback to this technique is that it depends on the ability of the designer to accurately predict the frequency of the vibrational modes. This becomes increasingly difficult in high accuracy regimes because the servo system is exposed to many unforeseen disturbances that excite unanticipated vibrational modes. For example, in a hard drive actuator such disturbances include servo amplifier saturation and distortion, external forces on the arm assembly, e.g., due to seek activity, air turbulence, stiction and the like. Such disturbances are typically generated at points in the control path where correction is impossible when gain stabilizing filters are present in the control loop. Consequently, although notch filters are useful in reducing predicted resonances of the servo control system, they do not inhibit the excitation of other vibrational modes by agents external to the servo control loop.
Another technique for damping vibrational modes of a servo control system was presented by Masahito Kobayashi et al. in xe2x80x9cMR-46 Carriage Acceleration Feedback Multi-Sensing Controller for Sector Servo Systems,xe2x80x9d at the International Conference on Micromechtronics for Information and Precision Equipment, Tokyo, Jul. 20-23, 1997. This proposed multi-sensing control system uses accelerometers to generate acceleration feedback An acceleration feedback controller receives the feedback signals and compensates the servo to eliminate the mechanical resonance modes.
Although Kobayashi""s technique has been demonstrated, it can not be efficiently implemented without the use of notch filters. Furthermore, designing the feedback controller requires the designer to model the very complicated transfer function Hd(s) of the servo-controlled system. This is computationally challenging and requires a considerable amount of processing time. In addition, because the poles and zeros of the compensator used in the feedback controller can not be predetermined, it is not possible to guarantee the existence of a stable compensator.
The prior art also teaches gain stabilization through low-pass filtering in the control loop. In this approach the cutoff frequency of a low-pass filter that is inserted in the control loop is generally lower than the frequencies of any of the lightly damped resonances of the actuator structure. Thus, the components of the control signal having the resonance frequency are effectively prevented from exciting the vibrational modes of the actuator structure. This helps ensure system stability, but it also increases the phase shift at frequencies in the vicinity of the servo loop""s unity gain crossing, thereby reducing the bandwidth of the servo system. In fact, this drawback applies to all gain stabilizing filters, including notch filters. The reduction in bandwidth, in turn, reduces the ability of the servo system to correct low frequency vibration and tracking performance such as run out and other disturbances that are due to external excitation and non-linearities in positioning operations.
In U.S. Pat. No. 5,459,383 Sidman et al. teach a feedback loop using a motion sensor located in the servo system at or near the point of control. The sensor is referred to as collocated because it is at or near the point of control. During operation the sensor detects both the rigid body motion and deformation of the actuator. The signal component from the rigid body motion is always much larger than that due to deformation. The collocated feedback loop operates Win conjunction with the ordinary feedback loop and has the effect of making the servo system perform as if the mechanical structure of the system had a much higher mechanical damping than it actually possesses.
Although Sidman""s system does improve positioning control and tolerance to internally and externally induced vibrational modes, it still relies on gain filters. Some negative effects of these filters are mitigated by the collocated feedback loop, but the most serious drawbacks, especially the requirement that the engineer know the vibrational modes ahead of time to ensure proper system design, are not obviated. Furthermore, the signal derived from the sensor includes the large rigid body component, which is also processed by the feedback loop and affects the rigid body motion properties of the actuator. This is undesirable since the rigid body properties of the actuator should be preserved.
Thus, the problem of stabilizing servo-controlled actuator systems remains. Solutions using filtering techniques are inadequate in high-accuracy regimes, e.g., in high density hard disk drives, since they require a priori knowledge of the vibrational modes of the system. Meanwhile, solving the transfer function to determine the vibrational modes is computationally unfeasible or impossible in most practical cases.
Finally, prior art systems suffer from the limitation of not being able to actively compensate for multiple vibrational modes at the same time. Specifically, if more than one single mode is selected for active control system stability is, at risk.
Accordingly, it is a primary object of the present invention to provide an active control mechanism for stabilizing a servo-controlled actuator system and overcome the disadvantages of the prior art. Specifically, the control mechanism of the invention is designed to circumvent the limitations of the gain filtering approach and provide effective feedback over the actuator""s entire range of operation by actively compensating for multiple vibrational modes, thus permitting higher bandwidth operation.
It is another object of the invention to design the control mechanism in such a way that no a priori knowledge of the system""s vibrational modes is required for the digital servo controller.
Still another object of the invention is to increase the bandwidth of stable operation of the servo-controlled system and to thus permit one to design, e.g., in the field of hard disk drives, devices with a higher number of tracks per inch (TPI).
Yet another object of the invention is to provide a method for operating the system of the invention to produce an efficient feedback signal. The method limits the computational effort and ensures that the system compensates vibrations quickly and reliably.
Finally, it is an object of the invention to circumvent the necessity of directly solving the transfer function.
The above objects and advantages, as well as numerous improvements attained by the system and method of the invention are pointed out below.
These objects and advantages are attained by an active control mechanism for stabilizing a servo controlled actuator system having an arm assembly, a controlled element mounted on the arm assembly, an actuator for moving the controlled element by moving the arm assembly and a position sensor, e.g., the controlled element itself, for generating a position signal indicating a displacement of the controlled element. The arm assembly experiences vibrational modes during operation, which are compensated by the active control mechanism.
The control mechanism has a sensing arrangement which can include one or more individual sensors attached to the actuator for generating signals correlated to and in phase with the vibrational modes, and in particular with all the major vibrational modes at low frequencies. A control mechanism is connected to the sensors to derive from their signals, an adjustment signal having three corrective terms a stiffening correction, an active damping correction and an inertia correction. The control mechanism also contains a phase shift circuit to shift the phase of the sensor signals to achieve the three corrective terms and to stabilize out of phase modes at high frequencies. A combining element connected to the control circuit, to the position sensor and to the actuator combines the adjustment signal and the position signal to produce a feedback signal. This feedback signal is used to drive the actuator via a current source, which is connected to the combining element and actuator. Typically, the current source simply delivers a current proportional to the feedback signal to the actuator.
In the preferred embodiment the actuator is a coil, e.g., a VCM coil, and the sensors detect the in-plane sway deformations of the coil produced by the major vibrational modes. Suitable types of sensors for use in the system include strain sensors, strain rate sensors and strain acceleration sensors. The preferred sensor type is a strain rate sensor. In this case the control mechanism induces a 90xc2x0 phase lag for generating the stiffening, correction, a linear element for generating the active damping correction and mechanism for inducing a 90xc2x0 phase lead for generating the inertia correction. Alternatively, a phase lag of between 90xc2x0 and 270xc2x0 may be used in thins case to stabilize out of phase modes.
The arm assembly typically has a coil support where the coil is mounted and the sway deformations of the coil produce deformations of the coil support which are registered by the sensors. In fact, for best results the sensors are mounted on the coil support.
In some applications, such as hard disk drives, the preferred type of actuator is a rotary actuator. In this case, of course, the controlled element is a read/write head. Other applications require actuators executing other than rotary adjustments, e.g., linear displacements, and employ other types of controlled elements.
Depending on the implementation, the control mechanism may require a low-pass filter for cleaning the signals received from the sensors from unwanted high-frequency noise components. Alternatively, a filter such as a low pass filter, notch filter or low pass filter with zero may be used to implement a phase shift. In addition, an on/off switch or similar circuit can be connected to the control mechanism to halt its operation if needed, e.g., to park the arm assembly on a load/unload ramp.
The method of the invention actively stabilizes the actuator assembly by relying on the signals that are correlated to and in phase with the vibrational modes. In practice, the vibrational modes consist of major modes and minor modes and the sensors have to generate signals correlated to and in phase with the major modes. This is ensured by the proper placement of sensors. The placement can be ascertained by an empirical or computational method. When phase correction is used, the phases of the signals are shifted with respect to modes.
In the empirical method the sensors are removably affixed to the actuator, or the coil support, at a test position. Next, the test position is adjusted until a final position or placement is reached at which the signals delivered by the sensors are in phase with the major vibrational modes. The sensors are then permanently attached at the final position. The computational method involves analyzing the vibrational modes of the actuator and determining the final position based on this analysis.
The first three correction terms are derived from the signals by shifting the phase of the signals. The operations required to derive each correction depend on whether the sensors used measure strain S, strain rate Sxe2x80x2, or strain acceleration Sxe2x80x3. In the preferred embodiment the sensors measure the rate of strain Sxe2x80x2 and the stiffening correction is derived by a phase lag of 90xc2x0, the active damping correction is derived by multiplying the signals by a constant, and the inertia correction is obtained by a phase lead of 90xc2x0. The phase shifting term is obtained from a phase-shifter circuit.
The particulars of the invention are explained in the description portion in reference to the appended drawing figures.